Abstract

This research focuses on the synthesis and characterisation of novel molecular salts with the potential to undergo spin crossover (SCO) transitions. SCO describes the reversible transition between the LS and HS states, which can occur in some first row transition metal complexes with d4 – d7 valence electrons, induced by changes in external stimuli such as temperature. The rearrangement of electrons during the SCO transition results in changes to the properties exhibited by the bulk material, which can be characterised by a number of methods. Such techniques used in this work include X-ray diffraction (XRD), SQUID (Superconducting Quantum Interference Device) magnetisation measurements, differential scanning calorimetry (DSC) and Mössbauer spectroscopy.
Both cationic and anionic complexes of iron (II) and iron (III) have been synthesised with a range of ligands, based on 2,6-bis(pyrazol-1-yl)pyridines/pyrazines (dpp), salicylaldehyde triethylenetetramine (sal2-trien) and salicylaldehyde thiosemicarbazones (thsa). Ionic materials previously reported to exhibit molecular conductivity are used as counterions, such as tetrathiafulvalene (TTF+), 7,7,8,8-tetracyanoquinodeimethane (TCNQ-), nickel (II) complexes of maleonitriledithiolate ([Ni(mnt)2]-) and 2-thioxo-1,3-dithiole-4,5-dithiolato ([Ni(dmit)2]-). Metathesis reactions or electrocrystallisation experiments have been utilised, resulting in salts in one of the following combinations; [SCO]+[Conductor]- or [Conductor]+[SCO]- where SCO is a potential SCO metal complex and conductor is an ion which may support electrical conductivity. Whilst there are a limited number of examples of the former, no examples have been found of the latter combination. The synthesis of SCO double salts has also been attempted to give [SCO][SCO] compounds, by reacting cationic iron (II) complexes with dpp ligands with anionic iron (III) complexes with thsa ligands. All products have been analysed by a range of techniques, including elemental analysis, infra-red spectroscopy, XRD, DSC, SQUID magnetisation measurements, Mössbauer spectroscopy and variable temperature resistance measurements.
A number of novel products have been synthesised and characterised, some of which exhibit interesting behaviours ranging from SCO transitions to semiconductivity to long range magnetic order. Iron (II) complexes with dpp-type ligands exhibited a range of SCO behaviours (hysteretic, abrupt, gradual) depending on the anion. A combination of iron (III) complexes with sal2-trien derivative ligands yielded both ferro- and ferrimagnetic compounds. It was determined that minor structural changes via halogen substitutions (Cl or Br) can change the nature of magnetic ordering. The presence of both iron (II) and iron (III) centres in the SCO double salts resulted in multiple SCO transitions.